Buggy code:
public class BcelTypeMunger extends ConcreteTypeMunger { public BcelTypeMunger ( ResolvedTypeMunger munger , ResolvedType aspectType ) { } public String toString ( ) { } public boolean munge ( BcelClassWeaver weaver ) { } private String getShortname ( String path ) { } private boolean mungeNewAnnotationOnType ( BcelClassWeaver weaver , AnnotationOnTypeMunger munger ) { } private boolean mungeNewParent ( BcelClassWeaver weaver , NewParentTypeMunger munger ) { } private boolean enforceDecpRule1_abstractMethodsImplemented ( BcelClassWeaver weaver , ISourceLocation mungerLoc , LazyClassGen newParentTarget , ResolvedType newParent ) { } private boolean enforceDecpRule2_cantExtendFinalClass ( BcelClassWeaver weaver , ISourceLocation mungerLoc , LazyClassGen newParentTarget , ResolvedType newParent ) { } private boolean enforceDecpRule3_visibilityChanges ( BcelClassWeaver weaver , ResolvedType newParent , ResolvedMember superMethod , LazyMethodGen subMethod ) { } private boolean enforceDecpRule4_compatibleReturnTypes ( BcelClassWeaver weaver , ResolvedMember superMethod , LazyMethodGen subMethod ) { boolean cont = true ; String superReturnTypeSig = superMethod . getReturnType ( ) . getSignature ( ) ; String subReturnTypeSig = subMethod . getReturnType ( ) . getSignature ( ) ; if ( ! ( superReturnTypeSig . equals ( subReturnTypeSig ) ) ) { ResolvedType subType = weaver . getWorld ( ) . resolve ( subMethod . getReturnType ( ) ) ; ResolvedType superType = weaver . getWorld ( ) . resolve ( superMethod . getReturnType ( ) ) ; <START_BUG> if ( ! ( subType . isAssignableFrom ( superType ) ) ) { <END_BUG> ISourceLocation sloc = subMethod . getSourceLocation ( ) ; weaver . getWorld ( ) . getMessageHandler ( ) . handleMessage ( MessageUtil . error ( ( ( ( ( "The<seq2seq4repair_space>return<seq2seq4repair_space>type<seq2seq4repair_space>is<seq2seq4repair_space>incompatible<seq2seq4repair_space>with<seq2seq4repair_space>" + ( superMethod . getDeclaringType ( ) ) ) + "." ) + ( superMethod . getName ( ) ) ) + ( superMethod . getParameterSignature ( ) ) ) , subMethod . getSourceLocation ( ) ) ) ; cont = false ; } } return cont ; } private boolean enforceDecpRule5_cantChangeFromStaticToNonstatic ( BcelClassWeaver weaver , ISourceLocation mungerLoc , ResolvedMember superMethod , LazyMethodGen subMethod ) { } public void error ( BcelClassWeaver weaver , String text , ISourceLocation primaryLoc , ISourceLocation [ ] extraLocs ) { } private LazyMethodGen findMatchingMethod ( LazyClassGen newParentTarget , ResolvedMember m ) { } public boolean attemptToModifySuperCalls ( BcelClassWeaver weaver , LazyClassGen newParentTarget , ResolvedType newParent ) { } private String createReadableCtorSig ( ResolvedType newParent , ConstantPoolGen cpg , INVOKESPECIAL invokeSpecial ) { } private ResolvedMember getConstructorWithSignature ( ResolvedType tx , String signature ) { } private boolean mungePrivilegedAccess ( BcelClassWeaver weaver , PrivilegedAccessMunger munger ) { } private void addFieldGetter ( LazyClassGen gen , ResolvedMember field , ResolvedMember accessMethod ) { } private void addFieldSetter ( LazyClassGen gen , ResolvedMember field , ResolvedMember accessMethod ) { } private void addMethodDispatch ( LazyClassGen gen , ResolvedMember method , ResolvedMember accessMethod ) { } protected LazyMethodGen makeMethodGen ( LazyClassGen gen , ResolvedMember member ) { } protected FieldGen makeFieldGen ( LazyClassGen gen , ResolvedMember member ) { } private boolean mungePerObjectInterface ( BcelClassWeaver weaver , PerObjectInterfaceTypeMunger munger ) { } private boolean mungePerTypeWithinTransformer ( BcelClassWeaver weaver ) { } private boolean couldMatch ( BcelObjectType bcelObjectType , Pointcut pointcut ) { } private boolean mungeNewMethod ( BcelClassWeaver weaver , NewMethodTypeMunger munger ) { } private ResolvedMember getRealMemberForITDFromAspect ( ResolvedType aspectType , ResolvedMember lookingFor ) { } private void addNeededSuperCallMethods ( BcelClassWeaver weaver , ResolvedType onType , Set neededSuperCalls ) { } private void signalError ( String msgid , BcelClassWeaver weaver , UnresolvedType onType ) { } private boolean mungeNewConstructor ( BcelClassWeaver weaver , NewConstructorTypeMunger newConstructorTypeMunger ) { } private static LazyMethodGen makeDispatcher ( LazyClassGen onGen , String dispatchName , ResolvedMember superMethod , BcelWorld world , boolean isSuper ) { } private boolean mungeNewField ( BcelClassWeaver weaver , NewFieldTypeMunger munger ) { } }

Fix:
if ( ! ( superType . isAssignableFrom ( subType ) ) ) {